1. Field of the Invention
The present invention is directed to an electric power distribution track for various power consuming loads such as lighting fixtures and other electric devices, and more particularly to an electric power distribution track of the type comprising a metal-reinforced plastic duct and at least three conductors which extend along the length of the duct at positions spaced inwardly from the inner periphery of the plastic duct and are held by respective supporting members integrally projecting on the top wall and the opposite sidewalls to form a trigonous arrangement of the conductors at any cross section of the duct.
2. Related Prior Art
Electric power distribution tracks having a metal-reinforced plastic duct are known, for example, as disclosed in U.S. Pat. No. 3,704,437, which comprises a metal-reinforced plastic section which carries several electric conductors on its inner periphery of the plastic duct. However, there is no provision of holding the conductors on the top wall and the opposite sidewalls of the plastic duct respectively by support members projecting inwardly from the insulation sheath of the duct.
In the meanwhile, the above trigonous arrangement of the conductors is advantageous in that the three conductors can be selectively utilized as forming a three-phase current distribution circuit or as forming two independent circuits carrying a single-phase three-wire current. Apart from the above, it is preferable for the metal-reinforced plastic duct including a metal core covered by an insulation sheath of the plastic material to separate the conductors from the metal core by a distance great enough to prevent accidental shorting therebetween due to possible defect or breakage of the insulation sheath as well as to elongate as much as possible the creepage distance of insulation at an exposed end face of the track.
To these ends, it may be reasonable to integrally project the supporting members holding the respective conductors in spaced relation from the inner periphery of the duct, or from the top wall and the sidewalls. However, this attempt certainly induces another problem that increasing the amount of projection of the support member results in the loss of rigidity of the support member and therefore the support member as well as the conductor held thereby become unstable to such an extent that proper contacting engagement is no more expected between the conductor and a cooperative terminal blade of an adaptor plug through which the electric device derives the power from the track, in addition to that, misalignment occurs between the conductors of the adjacent tracks at the time of coupling the lengths of the tracks.
One solution to the above is thought to provide suitable ribs each integrally bridging between the support member on the top wall and that on each of the sidewalls within the confines of the track for joining the three support members into a consolidated unit. But, there still remains a further problem in this scheme that the rib of a certain configuration, for example, the one extending straight will result in excess reinforcing effect to permanently retain the three support members in a fixed trigonous relationship even if the duct or track be subjected to such an external force to spread outwardly the sidewalls and the support member thereon as might be seen at the time of dropping the track onto the floor. This may adversely cause the unacceptable separation of the inner sheath at the top wall, from the core member to which it has been adhered, since the consolidated unit of three support members would act as a rigid construction member absorbing substantially no stress attendant with the above external force and consequently pull the insulation sheath of the top wall downwardly as the sidewalls and the support member thereon are forced to be deformed outwardly in a spreading manner. Such separation of the insulation would lead to the breakage of the insulation sheath and therefore should be unacceptable from the view point of providing an electrically safe track of practical utility.
3. Related Applications
A related application is application Ser. No. 694,852 filed Jan. 25, 1985 by the same applicants entitled "Electric Power Distribution Track" now abandoned.